The present invention relates generally to the field of electronics and, in particular, to automatic level control for an input to an analog to digital converter.
Linear Amplifiers, analog to digital (A/D) converters, and analog laser transmitters are used in a wide variety of communication circuits. For example, A/D converters are used in high frequency digital transmission systems that receive analog input signals. The A/D converter is an electronic circuit that receives the analog input signal and produces a digital output signal. The digital output signal is produced based on samples of the analog input signal taken over time and processed through a plurality of output registers. The A/D converter samples the digital signal based on a clock signal. In high-speed applications for digital transmission systems, these clock signals typically operate at speeds in the tens to hundreds of megahertz range.
A/D converters and other circuits, such as amplifiers and laser transmitters, typically found in communication circuits have a limited dynamic range in that the circuits are designed to process analog signals over a specified range of values. In an A/D converter, when the input signal exceeds the specified peak input signal level, the output registers of the A/D converter overflow. This occurs even when the input voltage exceeds the peak input level by a value that would cause the A/D converter output to exceed its maximum level by a single least significant bit. Further increases beyond this point produce spurious output signals (distortions) that are proportional to the degree of overload of the A/D converter""s output registers. Similar distortions are produced when an amplifier or a linear laser transmitter is driven beyond its optimum linear range (which is known as the compression point in linear systems). The spurious signals can be detrimental to the operation of the devices using the communication network. However, if this overload is kept at sufficiently low levels in terms of amplitude and frequency of occurrence, then the resultant distortion can be acceptable in many applications; but operating too far below the optimum drive point reduces the signal to noise ratio of the communication network and degrades the performance. Thus for best performance, it is necessary to operate near the optimum point while not exceeding it. Such an operation does not leave much room for overdrive condition necessitating expensive high linearity devices or automatic gain control mechanisms as described below.
To reduce the effect of overflow conditions, some electronic systems are adjusted to operate well below the peak input voltage range of the A/D converter and other electronic circuits with limited dynamic range. When initially setting up the system, the input voltage is increased gradually while the circuit is monitored, e.g., by an overflow register of the A/D converter. In the case of a typical AID converter, when an overflow condition is reached, the A/D converter provides an output of narrow pulses from the overflow register. The frequency of occurrence of the pulses is typically proportional to the severity of the overflow. Based on the monitored output of the overflow register, the input to the A/D converter is adjusted, e.g., by setting an attenuator, such that the expected maximum input signal will not exceed the peak input range of the A/D converter. In some systems, this adjustment is set at or near 50% of the peak input voltage level. Unfortunately, this reduces the effectiveness of the A/D converter and reduces the signal to noise ratio for the electronic device.
In some circuits, it is desirable to maintain the peak input voltage of the analog signal at or near the maximum value for the range of input signals accepted by the devices with a limited dynamic range, e.g., an analog to digital converter. Thus, automatic gain control circuits have been used in conjunction with circuits including analog to digital converters, amplifiers and laser transmitters. Typically, the automatic gain control circuit monitors the input to the device with the limited dynamic range. The automatic gain control circuit further generates a feedback signal based on the monitored input signal. The feedback signal is provided to an amplifier to control the level of the input signal to the analog to digital converter. This feedback signal attempts to keep the peak voltage level of the input signal at or near the full-scale value of the input for the analog to digital converter.
Conventionally, the feedback signal for A/D converters is generated using analog circuitry. For example, such feedback loops typically include one or more of the following analog control blocks: a log amplifier, a summing amplifier, an integrator, and a differentiator. Unfortunately, these analog feedback control loops typically suffer from the so-called xe2x80x9cclipxe2x80x9d effect. This means that when the input signal exceeds the full-scale input for the analog to digital converter, the control circuitry is unable to quickly reach steady-state operation. Some control loops attempt to use digital circuitry to overcome these problems.
Automatic gain or level control for circuits that have a limited dynamic range may introduce other problems into transmission systems. When automatic level control is used, total gain from system input to system output may vary over time as a function of input signal level. In some transmission systems, devices coupled to the transmission system expect a substantially constant or uniform gain from the system. For example, cable modems coupled to hybrid fiber/coax (HFC) networks include circuitry that is designed to monitor and adjust for changes in gain in the HFC network. Thus, as the automatic level control circuit attempts to compensate for an increasing input from a cable modem, the modem responds by further increasing its output provided to the network. Such a course of events can have disastrous consequences.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improvements in automatic level or gain control that extend the dynamic range of circuits without compromising the operation of associated circuits and systems.
The above-mentioned problems with automatic gain control in telecommunications systems and other problems are addressed by the present invention and will be understood by reading and studying the following specification. Embodiments of the present invention provide information on gain adjustments in circuitry associated with a transmitter to circuitry associated with a receiver to produce offsetting gain adjustments at the receiver. Advantageously, the use of this xe2x80x9cdual gain controlxe2x80x9d produces a channel with a substantially constant gain function while allowing the dynamic range of circuitry associated with the transmitter to be extended.
More particularly, in one embodiment a method for controlling gain in a network is provided. The method includes receiving signals for transmission over a network and adjusting the level of the received signals. The method further includes inserting an additional signal indicative of the level adjustment and transmitting the signals and the additional signal over the network. The method also includes extracting the additional signal after transmission over the network and compensating for the level adjustment based on the extracted signal.